Is “idle” time not considered?
Only folks from Fly.io can confirm, but this answer here by Greg has some pointers: Understanding FIRECRACKER LOAD AVERAGE.
Or is it that the VMs shutdown when not in use? If so, are there any “cold start” penalties?
No, there are not. That said, shared-cpu-1x is (?) ~1/15th CPU slice: shared-cpu-1x performance on CPU bound tasks.
Btw, “serverless” is a planned (future) feature with ~500ms cold starts, if Kurt is to be believed. Tease us with more "machine" info?
A AWS Community Hero deploying their personal blog on Fly.io? That’s a first 